Continuous corn can have impact on disease spectrum

With corn’s rise in popularity on southern row crop farms has also come an increase in disease pressure. This is especially true in continuous corn production systems.

In 2012, gray leaf spot, which has historically been a problem in the Ohio River Valley, was seen more regularly in Mississippi.

However, over the past few seasons the disease has been observed in many of the Mid-South’s continuous corn fields especially in situations utilizing reduced till or no-till production systems.

Because of the disease’s reliance on specific environmental conditions for development, no treatment threshold for gray leaf spot exists. Instead, a grower’s best option is to plant a tolerant hybrid in situations where continuous corn is planned.

Otherwise, the decision whether or not to treat with a fungicide is based on the presence of lesions and their placement within the crop canopy. The area from the ear leaf and above should remain disease free.

Once an ear has been set, these are your most important leaves from a yield standpoint, according to Mississippi Extension Plant Pathologist Tom Allen. Treatment with a broad-spectrum fungicide labeled for the disease may be necessary if the field in question has a past history of gray leaf spot disease pressure.

To identify gray leaf spot, growers should be on the lookout for the disease’s characteristic tan-colored lesions, typically one-eighth of an inch in length when the lesions are at their youngest and growing to a maximum of one-inch in length.

The lesions exhibit parallel margins and do not cross the veins in the leaf and typically have a faint yellow halo around the outside if you hold the leaf up to the sunlight.

In addition, you should look on the underside of the leaf with a hand lens for little black spots in a row within the lesion itself.

The fungus that causes gray leaf spot can only infect corn plants through natural openings. Black spots are characteristic of gray leaf spot since they signify the fungus infecting the corn leaf through the stomates.

Lesions on the leaf can be confused with other diseases as well as abiotic disorders such as drought stress, herbicide injury, as well as nutrient injury and adjuvant burn, says Allen.

Another group of corn diseases that occurred during the 2012 season were corn rusts. There are two distinct types of corn rust — common rust and southern rust.

Common rust is generally an earlier season disease since the fungus prefers cooler temperatures, while southern rust is a late-season disease and can survive higher temperatures. Neither disease overwinters in the Mid-South or the U.S. The disease, instead, must blow in from other areas to the south for infection to occur.

Common rust traditionally has not been a disease that requires a fungicide due to low incidences of disease pressure and the disease’s aversion to high temperatures. Common rust, according to Allen, is typically identified by its cinnamon color pustules that may be located both on the top and on the bottom of the leaf.

Southern rust, conversely, prefers high temperatures and can be more damaging depending when infection occurs during the growing season. The worst possible time for infection to occur in a corn crop is shortly after the tasseling stage.

Southern rust is more orange in color than its common counterpart, and the rust pustules are typically only present on the top of the leaf. Fungicide management may be necessary depending on the level and timing of infection. Only one resistant hybrid is currently commercially available, and it is mostly planted in limited acreage in the southeastern United States.

While the incidence of common rust also is of little to no concern to Southeast corn growers, southern corn rust has been a problem in Georgia this year, according to University of Georgia Plant Pathologist Bob Kemerait.

Aided by tropical storms that brought with them ample amounts of rain, the disease blew into the area early enough prior to harvest to trigger fungicide applications, and is especially affecting Georgia’s later maturing corn crop.

“The majority of our corn acreage with any decent yield potential has been sprayed with a fungicide. In a year like this, we continue to apply fungicides because our evidence shows that protecting the crop remaining in the field is a decision that will pay off,” says Kemerait.

While southeastern corn producers have traditionally relied on the southern corn rust tolerance provided by Pioneer 3352 to combat the disease, that strategy is no longer enough, according to Kemerait.

“It has held up extremely well to our southern rust, but over the past several years there has been an introduction of one new race of the fungus and tolerance is again an issue,” he says. “It’s still an important tool for rust, but the hybrid is susceptible to other diseases, such as southern corn leaf blight.”

The region’s growers also battled two major disease issues throughout 2012 —northern corn leaf blight and southern corn leaf blight. While northern corn leaf blight is a year-in, year-out problem for southeastern producers, southern corn leaf blight has only recently re-emerged as a major concern for the region’s growers.

More than 40 years have passed since the corn production season of 1971 — which Kemerait says was a “huge problem year” for southern corn leaf blight.

“For a long time, it hasn’t been a problem, and we have rarely seen it in any appreciable amount until this year.” The disease was reported “very aggressively” attacking later maturing corn, especially that portion of the crop that was in the tasseling or silking stages of growth.

Because the disease has come on strong during the late season, Kemerait suggests that those growers with later planted, potentially high yielding corn should give strong consideration to an earlier fungicide application instead of waiting until the tassle stage to spray. Ideally, he says, a fungicide should be applied at the V6 growth stage.

“The shorter the crop rotation out of corn production the more I expect the incidences of northern and southern corn leaf blight will increase.” The multiplying effect is likely amplified in continuous corn production systems.